horizontal component of the velocity of the stone remains constant  $20 \cos \theta =10\sqrt{3}m/s$

but initial vertical component of velocity is $20\sin {30}^0=10m/s$    

 and final vertical velocity  will be $v_y=\sqrt{{\left(u_y\right)}^2+2gy} =\sqrt{{10}^2+2\left(10\right)\left(15\right)}=20m/s$

resultant velocity = $\sqrt{{\left(10\sqrt{3}\right)}^2+{20}^2}=\sqrt{700}=10\sqrt{7 }m/s$